Liam Fox (front) and Adam Werritty (far back, in purple tie) on the official trip to Sri Lanka. Photograph: British High Commission

The most senior civil servant in the Ministry of Defence had previously warned Liam Fox about his connections to a self-styled adviser and personal friend, Adam Werritty, the Guardian can reveal.

Officials had also previously told Fox to stop Werritty handing out business cards that falsely describe him as an "adviser to the Rt Hon Dr Fox MP".

On Thursday, Fox said he was referring the matter to Ursula Brennan, permanent secretary at the Ministry of Defence, to investigate "baseless accusations" that the friendship had led to breaches of national security.

One source said Brennan spoke to Fox about Werritty's role in the meeting in Dubai this summer on 19 August – the day the Guardian revealed Werritty's role in the meeting, which has left Fox facing the prospect of being called to give evidence in a blackmail trial in the US.

"They have offices next to each other, they discuss any number of matters every day – she would have mentioned this on the day your story came out," the source told the Guardian.

The MoD did not respond to requests to detail how often Brennan had spoken to Fox about Werritty. The department also failed to respond to claims that the officials had earlier told Fox to stop Werritty handing out the business cards, which are embossed with a House of Commons portcullis logo.

This latest revelation comes as Fox said it was "unacceptable" that Werritty had distributed the business cards. "I understand those cards are no longer used. I have made it very clear to him that it's unacceptable to carry a card saying that he is a personal adviser."

On Thursday Fox asked Brennan to launch an investigation into what he called "wild allegations". "Because there have been some allegations of security leaks and so on, I've asked the permanent secretary to look into that for me," he told the BBC. "I've asked the permanent secretary to look into any of these wild allegations, and I'm very happy to stand by that investigation."

Brennan will investigate whether Fox has breached the ministerial code.

Kevan Jones, a Labour MP and former armed forces minister, said it was "unthinkable" that the permanent secretary would not have already raised concerns about Werritty. "She would have raised it," Jones told the Guardian. "She is there to protect the reputation of the department. And also to protect the secretary of state from any conflict of interest."

It came a month after the pair went to Dubai to meet Harvey Boulter, the chief executive of Porton Capital, a private equity partner of the government which is embroiled in a legal case over a threat to withdraw a knighthood awarded to the British chief executive of the US Post-it note maker 3M.

The meeting, at which no government officials were present and of which no records were kept, is at the heart of a pending US blackmail trial.

Fox has been told that he will be requested to attend the trial if it reaches court.